Abstract: A system and method for determining the addition or removal of an object from a receptacle is provided. An ultrasound sensor may emit one or more bursts of ultrasonic energy and detect the amplitude of the echoes from each respective burst over time. A microcontroller may determine a decay rate of each respective burst from the amplitudes of the respective echoes. The decay rates are compared for changes within the receptacle caused by reflection, transmission, absorption, and diffusion of the ultrasound waves. If the difference between the second and first decay rates is larger than a predetermined amount, the addition of an object is determined.